The present invention relates to a current output circuit, and more particularly, to a bipolar transistor circuit which outputs a current of a polarity determined by input signals.
Such a circuit is employed as a charging/discharging control circuit of a load capacitor, for example. More specifically, when input signals take a first combination of voltage levels, the circuit outputs a current of one polarity which, for example, charges the load capacitor, and when the input signals take a second combination, the circuit outputs a current of the opposite polarity having the same amplitude which, for example, discharges the load capacitor.
According to prior art, such a circuit includes first, second and third current sources and first and second switches. The first current source is connected between one of power terminals and an output terminal to which a load is connected. The first switch and the second current source are connected in series between the output terminal and the other power terminal, and the second switch and the third current source are also connected in series therebetween. Each of the first to third current sources produces a reference current of the same value, and first and second switches are controlled by input signals, respectively.
When the input signals take the first combination of voltage levels, the first and second switches are turned OFF. Accordingly, the current produced by the first current source is supplied to the output terminal as a positive current. When the input signals take the second combination of voltage levels, both of the first and second switches are turned ON. At this time, the current of the first current source is counteracted by that of the second current source, so that the current of the third current source is supplied to the output terminal as a negative current. When the input signals take the third combination of levels, only one of the first and second switches is turned ON. The current of the first current source is counteracted by the current of second or third current source and the output terminal is supplied with no current.
For a high speed operation, each of the switches is favorably composed of a bipolar transistor. The bipolar transistor requires a base current to take a conductive state, and hence its collector and emitter currents are different from each other. For this reason, the current of the first current source is not counteracted completely. Assuming that the value of the current of each current source is I and the value of the base current I.sub.B, the positive output current has an amplitude of I and the negative output current has an amplitude of (I-2I.sub.B). When only one of the first and second switches is turned ON, the output terminal is supplied with a small current having an amplitude of I.sub.B.
Thus, the prior art circuit has unbalanced output currents of opposite polarities.